It has been known that the properties of photographic images can be remarkably improved by releasing a photographically useful reagent imagewise while silver images are formed.
One example of such an approach is the use of a DIR coupler in color photographic light-sensitive materials. A DIR coupler undergoes a coupling reaction with an oxidation product of a color developing agent upon development to release a development inhibitor, improving the graininess of color images, the sharpness by an edge effect and the color reproducibility by the dispersion of the development inhibitor into other layers. The details of such a DIR coupler are described in U.S. Pat. Nos.3,227,554 and 4,248,962, JP-B-58-9942 and JP-B-51-16141 (The term "JP-B" as used herein means an "examined Japanese patent publication"), and JP-A-52-90932, JP-A-56-114946, JP-A-57-154234, JP-A-58-188035, JP-A-57-151944 and JP-A-58-217932 (British Patent 396873) (The term "JP-A" as used herein means an "unexamined published Japanese patent application").
In recent years, it has been known that the use of a coupler which undergoes coupling reaction with an oxidation product of a color developing agent to release a competing compound, development inhibitor or fogging agent improves the graininess of color images or the sensitivity as described in British Patent 1,546,837, U.S. Pat. No. 3,408,194, and JP-A-57-138636, JP-A-57-150845 (U.S. Pat. No. 4,390,618), JP-A-59-50439, and JP-A-59-170840 (U.S. Pat. No. 4,628,024).
A colored coupler having an azo dye portion as an eliminatable group undergoes a coupling reaction with an oxidation product of a color developing agent to imagewise release an azo dye which then flows out into the processing solution. As a result, the colored coupler having an azo dye portion is left counterimagewise to give a masking effect for the improvement in color reproducibility. This is further described in JP-A-51-26034 and JP-A-51-110328, U.S. Pat. No. 4,049,929, and British Patents 1,443,875, and 1,464,361.
Thus, these functional couplers release a photographically useful reagent contribute to improvement in the image quality of color images and sensitivity. However, these functional couplers release a photographically useful reagent only by a coupling reaction with an oxidation product of a color developing agent. Therefore, these functional couplers have the fundamental disadvantage that they cannot be used in the field of light-sensitive materials which do not use a color developing agent, i.e., black-and-white photographic light-sensitive materials or diffusion transfer photographic materials. These functional couplers have another disadvantage that they produce an azomethine dye which has a bad effect on color reproducibility. DIR couplers as described in JP-A-49-77635 and JP-A-50-20725 or dye-discharging type couplers as described in JP-A-59-168444 have been proposed to eliminate such a disadvantage. However, these couplers remain insufficient. For example, these couplers have a low coupling activity or cause a remarkable contamination in the processing solution.
Extensive studies have been made to develop a redox compound which eliminates the disadvantages of these functional couplers and releases a photographically useful reagent regardless of the type of the developing agent used. However, it must such that redox compounds so far developed require further improvement.
For example, known redox compounds include DIR hydroquinones as described in JP-A-49-129536 (U.S. Pat. No. 3,930,863), and U.S. Pat. Nos. 3,379,529, 3,620,746, 4,332,878, and 4,377,634; DIR aminophenols as described in JP-A-52-57828 (U.S. Pat. No. 4,108,663); p-nitrobenzyl derivatives as described in EP 45,129; hydrazine derivatives as described in U.S. Pat. No. 4,684,604, and redox compounds having at least one carbonyl group as described in JP-A-61-213847. However, many of these compounds cannot release a photographically useful reagent from the oxidation product as fast as practically required. Even compounds which can release a photographically useful reagent as fast as required exhibit a poor storage stability. Thus, it has been difficult to develop a redox compound with both rapid function and excellent storage stability.